Thermal Mass

How to make a naturally comfortable environment in your house.

Old buildings are great because they are built from stone and brick that gives them "thermal mass". The Romans and Greeks exploited this 2 or 3 thousand years ago. Buildings with thermal mass feel cooler on hot days and feel warmer on cold days. They generally feel more comfortable. It is hard to explain how thermal mass works, but let me give it a go.

Materials such as stone, brick, and concrete absorb and store heat. They also absorb and store cold. The absorption process takes a long time and the release process also takes a long time. In winter the warmth of the day is released at night and in the summer the cool of the night is released during the day.

The thermal mass works best when it is within the thermal envelope of the house. By that I mean if the walls are concrete then the house insulation should all be on the outside of the concrete. You want the thermal mass to be directly coupled to the inside living space. If the concrete is also accessible to the outside then the heat will be lost to the outside.

The thermal mass can be just heated by the air inside the house, but even better is if you can let sunlight shine on it through the windows as the suns rays will heat it and you will be able to capture that free heat.

For it to work best you want to try to avoid screening the concrete walls with curtains and drywall etc.

You ideally only want to capture the heat from the sun in winter rather than summer. This can be done using appropriately designed roof and balcony overhangs because the sun in much higher in the sky in the summer compared with the winter.

To capture the most heat from the sun you want windows with a high solar gain. These are actually quite difficult to find because most windows are coated to reflect the sun's rays away because they figure that most house designs do not provide proper summer sun screening so would get too hot if a high solar gain was used.

There are lots of things that can provide thermal mass within your home, ie within your thermal envelope, eg...
    Concrete walls
    Brick internal walls
    Stone or brick big fireplace
    Stone or tile floors
    Rammed earth internal walls
    Tanks of water

The thermal mass effect works particularly well where there is a big difference between the temperature during the day compared with during the night.